We propose to employ intact micro-organisms in conjunction with ion and gas sensing membrane electrodes to devise bio-selective sensors and probes suitable for analytical measurements. By immobilizing living bacterial cells, in various experimental arrangements, at potentiometric membrane electrodes, we hope to be able to achieve sensors with improved stability, selectivity, and sensitivity. Furthermore, we expect that the use of living microorganisms will permit the development of sensors with response to substances not presently accessible to conventional electrodes as well as sensors with the capability of being regenerated during use. In this proposal, we outline the particular systems to be investigated, some new concepts for the physical design of bacterial sensors, steps for the culturing and modification of bacterial strains to achieve optimum electrode selectivity, and experiments to be conducted for the evaluation of experimental variables and operating parameters of the resulting sensors.